Natural aerosols (e.g., dust, sea salt, volcanic ash) and anthropogenic aerosols (e.g., particulate matter from fossil fuel combustion) perturb the earth's climate through direct effects (scattering or absorbing solar radiation) and indirectly by modifying cloud properties. However, the uncertainty in aerosol radiative forcing is high, and the ability to frame effective climate change policies depends on reducing this uncertainty by a factor of 3 to 10. Achieving this goal will require improvements in measuring the scattering and absorption properties of aerosols, especially in regions that are difficult to characterize with existing instruments (e.g., in or near clouds and coarse particle-dominated regions).
Light scattering due to aerosols increases rapidly at humidities above 90%. While measurements of humidified scattering have been made with wet nephelometers at relative humidities (RH) of up to 85%, measuring scattering at higher RH has proven technically challenging. Cavity ringdown spectrometers (CRDSs) have been used to quantify aerosol extinction (scattering and absorption) in humidities up to RH≈90%. But at very high humidities, as found in and near clouds, existing CRDSs perform poorly, diverging significantly from theoretically predicted extinction coefficients. It is difficult to maintain humidities greater than 90-95% in CRDS without condensational losses. In addition, significant loss of larger particles can occur due to impact losses with CRDS inlet tubing. Furthermore, humidity may be enhanced for tens of kilometers around clouds, and 30 to 60% of the global areas that are considered cloud-free may in fact include regions of high humidity. Because most satellite retrievals tend to be biased toward data that is far from clouds, some fraction of these “twilight zones” may not be included in existing satellite retrievals, which may degrade the accuracy of aerosol direct forcing estimates.
In situ measurements of aerosol extinction obtained during atmospheric profiles could help constrain ground- and satellite-derived aerosol optical depth (AOD) retrievals. The need for such constraints is acute not only in and near clouds but also in regions with high coarse dust burdens (e.g., continental-scale regions downwind of north African dust plumes). Such regions constitute an important source of uncertainty in estimates of radiative effects. Accordingly, there is a need for an instrument capable of making in situ aerosol extinction measurements in dusty and highly humid environments.
There is also a need for an instrument capable of characterizing gases that are too reactive or otherwise unsuitable to be measured with a closed instrument. Instruments used to measure particles and gases are usually calibrated and zeroed with calibrated or clean air. But some measurements need to be made in the open; in such instances the sample cannot be plumbed into an instrument. Calibrations and zeros are not easily achieved when the instrument is open to the ambient atmosphere. However, without calibrations and zeros, it is difficult to distinguish real changes in the sample of interest from artifactual changes in the reflectivity of the mirrors, windows, or other changes in the system.